This invention relates to a new process for protecting mineral samples. A typical field for the application of this invention is in the sampling and testing of core samples from oil and gas and other types of wells. Although not limited to this use, the invention is described with respect to this use. In drilling oil, gas and other types of wells, it is conventional practice to take samples of the strata through which the drill bit is passing. By analyzing these samples with respect to such parameters as permeability, porosity, and fluid saturation, a great deal can be learned regarding the nature of the particular strata or horizon from which the core was taken. These tests are generally included in the generic term "core analysis" which allow the characteristics of a particular reservior to be evaluated. Moreover, when combined with the same procedures in adjacent and distant wells this analysis provides a valuable aid in mapping the subsurface geology of a region.
The constant aim in obtaining core samples for subsequent analysis is to employ methods and techniques in the field that will ensure to the greatest possible extent that the core sample will be possessed of its original properties as it reaches the laboratory for analysis. It is evident that any intervening loss in moisture or oil content as well as any physical alterations, particularly those due to chemical interaction between the sample and the preservative material or inordinant temperature changes will result in erroneous interpretation of the characteristics of the subsurface strata from which the core was taken. Current methods do not provide a preservation process utilizing materials which provide satisfactory oxygen and water vapor barriers. The present invention is directed in part to improving techniques for core sampling in order to increase the reliability of data obtained therefrom. The method disclosed in the present invention utilizes materials which provide improved barrier properties, particularly against oxygen and water vapor transmission.
Different methods of core sample protection are known. A common method of protecting samples is to wrap the core first in polyvinylidene chloride (saran wrap) and then in aluminum foil. The foil is closed by crimping the edges and then the covered core is dipped in melted plastic such as B-60, which hardens into a seal. B-60 is a type of "Hot Melt Peel Coat Type II" made by Evans Manufacturing, Inc., in Warren, Mich. This method of protecting core samples is inadequate because the saran wrap is not chemically inert to the sample and disintegrates, oxygen and water vapor pass through the crimped seal, and the foil is readily ripped or punctured. Therefore, when using this preservation method, the only material which can be considered as a preservative is the plastic coating. At best, the plastic coating acts as a poor oxyben and water vapor barrier and B-60 is not chemically resistant to the core sample.
Another method of core sample preservation is to cast the sample in a plastic jacket as disclosed by Bailly in U.S. Pat. No. 2,662,401. This method requires the addition of a catalyst and accelerator to a heated liquid plastic. Coordination of gel time, the time necessary to cast the sample and the temperature rise in the plastic while gelling makes this method complicated and undesirable to use in the field. Furthermore, if the gel time is too short, stresses and strains are created causing cracking of the plastic, thereby defeating the preservative purpose of the plastic. Moreover, although Bailly teaches a plastic material possessing the desired qualities of chemical inactivity to the sample, oil and water barrier properties, and non-adsorbency of the sample gases, Bailly does not teach that the plastic material act as an oxygen barrier against oxygen entering the sample from the exterior. Bailly also does not teach the use of a layered or laminated material.